Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings, for example, typically comprise inner and outer raceways and a plurality of rolling elements (for example balls and/or rollers) disposed therebetween.
Bearings are used in a wide number of applications and it is often essential to be able to measure the relative orientation or rotational speed of the parts. For example, an ABS system can operate by measuring slip or stoppage of the surfaces within a bearing, compared to the expected rotation rate. A variance from the expected rotational speed would demonstrate a loss of traction and engage the ABS system.
It is known to use magnetic sensor systems in bearings to determine the relative orientation. An example of such a bearing assembly is shown in FIG. 1 which has a magnetic ring (a “tone” ring) attached to a portion of a rotating part of a bearing. The tone-ring is attached to the rotating surface in an off-centre manner, as shown in FIG. 2. Since the sensor is stationary, the magnetic field strength that it records will vary with the relative overlap between the sensor and the ring. This allows for accurate measurement of the rotational orientation of the relative parts of the bearing. In this way the number and frequency (RPM) of rotation can be determined.
The use of a tone ring, or an equivalent, such as a magnetic seal or disc (which relies on so-called variable reluctance) is well known. Indeed, the technology is commonplace in wheel speed sensors (ABS) for cars, trucks, and some railway applications. It is also commonplace for combustion engine crank and cam shaft position detection and in steer units, electric motor control and commutation sensor bearings.
The magnetic rings which are used in these known applications are typically manufactured in one of two different methods. According to the first method a magnetic material is sintered to form a ring. According to the second method, particles of magnetic material are mixed with polymers or elastomers, and then pressure moulded into a ring shape and, if necessary, vulcanised. The resulting rings are magnetised into the number of pole pairs needed. Typically, the polymers or elastomers are selected for their ease of mounting and to improve the mechanical strength of the ring.
The magnetic materials commonly used vary from simple ferrites (a form of magnetite, using barium or strontium to improve their qualities), to more expensive rare-earth-metal-based powders. Examples of suitable magnetic materials are well known in the art and include, for example, Neodymium-Dysprosium-Iron-Boron, Samarium Cobalt, Samarium Iron Nitride (SmFeN), or amalgamations of SmFeN and FeCo.
U.S. Pat. No. 5,200,697 discloses a bearing assembly having a tone ring for sensing the speed of rotation in a bearing.
It is an object of the present invention to provide an improved bearing assembly and to address some of the problems associated with the prior art, or at least to provide a commercially useful alternative thereto.